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Re: [Xen-devel] Supported #of CPUs/VMs per CPUs

To: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Subject: Re: [Xen-devel] Supported #of CPUs/VMs per CPUs
From: "George Dunlap " <dunlapg@xxxxxxxxx>
Date: Fri, 22 Sep 2006 09:45:35 -0400
Cc: Florian Kirstein <xenlist@xxxxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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100 virtual machines seems on one box seems like a lot to me. :-)

I beleve the Potemkin Virtual Honeyfarm project
(http://www.cs.ucsd.edu/~savage/papers/Sosp05.pdf) did some mods to
allow the numbers to go up into the multiple thousands.


On 9/22/06, Petersson, Mats <Mats.Petersson@xxxxxxx> wrote:

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of
> Florian Kirstein
> Sent: 22 September 2006 01:34
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] Supported #of CPUs/VMs per CPUs
> Hi,
> > There is no maximum number of domains per cpu.  The maximum
> number of
> > domains *per machine* (regardless of the # of CPUs) depends on the
> > amount of resources available, mainly memory.
> > I've never tried to max out the numer if idle domains in Xen, but
> > in general, the answer is quite a bit.
> I did try it out (see "maximum Number of DomUs" posting a few months
> back) on a 16 GB Quad-Opteron, and was surprised hitting a limit
> way before I used up all my memory, at about 107 DomUs. Xen has a
> private heap of 16MB and each Domain needs some memory there, so
> (possibly depending of what else is stored there) at about 100 DomUs
> that's it.

And for x86_64 there is no reason you can't change this limit, ether by
using the command-line argument, as I described in the post at that
point, or changing the parameter in the relevant header-file (also
described at the same time). There's really no reason why you can't
change this if you need to increase beyond this number of domains.

For 32-bit guests, it's a compromise, because the address space is
cramped with "only" 4GB of space, because the special heap needs to be
accessable by Xen at all times, thus mapped into all domains. This means
that the larger it is, the more memory space is occupied by the heap,
the less there is available for other things... I'm sure Keir or someone
will be able to give more details of why it is a bad thing to make it
much larger in 32-bit (including PAE, since PAE is only allowing the
entire system to have more than 4GB of memory, but at any given time,
the addressable space is 4GB, and the Xenheap must be mapped to ALL

In .../xen/include/asm-x86/config.h:
XENHEAP_DEFAULT_MB    - Number of mbytes of heap. In 32-bit, this is 12
from DIRECTMAP_MBYTES, and 16 for x86_64.

Command line paramaeter:

> Sure, not a real problem (there are few usefull szenarios in which
> one would want 100 DomUs on one host), but worth mentioning IMHO...
> Or did something change in this respect?

It's been like that for some time, I don't know how long tho'. There is,
as far as I understand, no STRICT limit for these things, it's just
about compromises between one benefit and another.

> (:ul8er, r@y
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